A molecular stuffing method is known as a process for producing a glass product having a gradient of refractive indices, in which a porous glass is used and its pores are filled with dopants (refractive index correction components).
Japanese Patent Application (OPI) No. 126207/76 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), laid opened on Nov. 1, 1976, describes a process for producing a glass product having a gradient of refractive indices by the molecular stuffing method, as well as a process for producing a porous glass. According to the process, a borosilicate glass is subjected to a heat treatment at a prescribed condition to separate it into an SiO.sub.2 -rich phase and an alkali metal oxide and B.sub.2 O.sub.3 -rich phase (phase separation). The glass is then treated with an aqueous solution containing an acid such as hydrochloric acid, sulfuric acid, or nitric acid, to leach the alkali metal oxide and B.sub.2 O.sub.3 -rich acid-soluble phase, thereby forming a porous glass having continuous micropores which has the SiO.sub.2 -rich phase as a skeleton. After allowing a solution of dopants to permeate into the micropores present in the porous glass thus prepared (stuffing), a part of the dopants is leached out from the outside of the glass product (unstuffing) to form a gradient of dopant concentration. The dopants are then solidified in the pores and the glass product is dried and subjected to a heat treatment to collapse the micropores, thereby obtaining a glass product having a gradient of refractive indices. Oxides, nitrates, carbonates, acetates, phosphates, borates, arsenates and silicates of alkali metals, alkaline earth metals, boron, aluminum, lead, phosphorus, and the like are described as examples of dopant.
A transparent glass product having a gradient of refraction is used as rod microlenses for microlens arrays, pickup lenses for optical video or audio disks, and the like. Rod microlenses used for rod microlens arrays of copying machines must have a .DELTA.n (difference of refractive index between central and peripheral portions of the lens) of only from about 0.007 to about 0.008. Therefore, the glass product obtained by the molecular stuffing method can sufficiently meet the requirement. However, lens arrays for facsimile and lens arrays for LED printers which are required to have a large numerical aperture and which are bright must have a .DELTA.n value exceeding 0.04 to 0.03, respectively. Therefore, a glass product satisfying this requirement cannot be obtained by merely utilizing the molecular stuffing method.
In order to increase the .DELTA.n by the molecular stuffing method, the concentration of dopants in the stuffing solution must be increased or a dopant having a high refractive index must be used. However, the concentration of dopants in the stuffing solution cannot be increased infinitely due to the limit of their solubility. For example, CsNO.sub.3, which is often used as a dopant, has a solubility at 100.degree. C. of at most 200 g/100 ml H.sub.2 O. Accordingly, even if a saturated solution of CsNO.sub.3 is used as a stuffing solution, no significant effect can be expected to increase the .DELTA.n. In fact, Japanese Patent Application (OPI) No. 126207/76 shows examples which produce a glass product having a gradient of refractive indices by stuffing a CsNO.sub.3 aqueous solution containing 120 g of CsNO.sub.3 per 100 ml of water at a temperature of 95.degree. C. wherein the maximum value for .DELTA.n is merely 0.031. A glass product having such a .DELTA.n value cannot be used in lens arrays for facsimile machines.
Regarding the use of dopants having a high refractive index, viz., dopants other than alkali metal compounds, even if those can be used for stuffing into the pores in a porous glass product, such dopants tend to react with the glass during heat treatment of the porous glass product, thereby making the glass product unstable. For example, Pb(NO.sub.3).sub.2 is known as a dopant to increase the refractive index and has a solubility of about 130 g/100 ml H.sub.2 O at 100.degree. C. Even if this is used for the stuffing of a porous glass, the porous glass becomes unstable during the heat treatment and a transparent glass product is hardly obtained. Accordingly, when a dopant having a high refractive index is used, the amount used thereof must be limited to the range where a transparent glass product can be obtained. In the case of Pb(NO.sub.3).sub.2, the concentration of the stuffing solution is at most about 40 g/100 ml H.sub.2 O and even if the stuffing is conducted with such a concentration, the refractive index at the central portion of the resulting glass product is at best 1.465, as is described in Japanese Patent Application (OPI) No. 126207/76.